Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on TCP/IP v4 network B). Assume that no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D?
- TTL
- Checksum
- Fragment Offset
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Answer: D
When an IP datagram travels from Host A to Host B across different networks, several fields in the IPv4 header can be modified by intermediate routers:
- TTL (Time to Live): Every router that forwards the datagram decrements the TTL field by at least to prevent routing loops. Thus, the TTL value at Host B will be less than the original TTL set by Host A.
- Checksum: Since the TTL field (and potentially other fields like fragmentation flags/offsets) changes at each hop, the IPv4 header checksum must be recomputed and updated by every router along the path.
- Fragment Offset: If the datagram encounters a physical network link with a Maximum Transmission Unit (MTU) smaller than the datagram's size, intermediate routers will fragment the datagram (unless the "Don't Fragment" bit is set). The resulting fragments reaching Host B will have different Fragment Offset values compared to the original unfragmented datagram (which had an offset of ).
Therefore, all three fields—TTL, Checksum, and Fragment Offset—may differ from those of the original datagram when it reaches Host B.
Hence, the correct option is D.
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